The present invention relates to imaging. In particular, the present invention relates to exposure correction in imaging devices, such as scanners of various types.
Scanners are used to receive and process an image from media into an electronic file format. High-precision scanners typically use a stepper motor to drive the media relative to the scanning sensor, or to drive the sensor relative to the media. Stepper motors have the benefit of precise motion, but are expensive and generally bulky. As consumers continue to demand a decrease in the size and cost of scanners, manufacturers have substituted higher-torque DC motors. The motion of the DC motors can be irregular, and such irregular motion results in inconsistent or non-uniform exposure of the media to the scanner light source.
Newer scanning devices utilize LEDs to illuminate the media for the scanning process. Because of their fast switchability, LEDs allow for a wide range of exposure adjustment possibilities through the use of feedback loops in the scanning circuitry. These scanning devices can turn the LEDs on and off to adjust for irregularities in the appropriate amount of exposure to a sensor array.
Many scanners utilize the uniform light offered by a cold cathode fluorescent lamp (CCFL) assembly. These lamps do not have rapid switchability. In order for the exposure under such lamps to be consistent, the amount of time that the media is exposed must be the same for all of the scan lines that are captured. Without the use of an accurate stepper motor, the varying motor speed will result in streaks on the scanned image due to non-uniform exposure of each scan line.